A primary claim of the hydraulic fracking industry is that deeply buried rock layers will always seal and contain the dangerous chemicals that are injected thousands of feet underground.

But a new study from Duke University concluded that fracking for natural gas under Marcellus Shale in Pennsylvania may lead to harmful gas or liquids flowing upward and contaminating drinking-water supplies.

The study found that salty, mineral-rich fluids deep beneath Pennsylvania’s natural gas fields are seeping upward thousands of feet into drinking water supplies. Although it found no evidence of fracking chemicals doing the same, the findings suggest that there are paths that would let hazardous gas or fluids flow up after drilling:

“The biggest implication is the apparent presence of connections from deep underground to the surface,” Robert Jackson, a biology professor at Duke and one of the study’s authors, told ProPublica. “It’s a suggestion based on good evidence that there are places that may be more at risk.”